WO 2008/121070 PCT/SE2008/050367 SCREW-ROTOR MACHINE, ENERGY-CONVERSION SYSTEM AND METHOD FOR ENERGY CONVERSION Field of the Invention The present invention relates, in a first aspect, to a screw-rotor machine, 5 preferably an expander, which machine comprises two screw rotors and a housing having an inlet opening and an outlet opening, which housing forms a working space in which the screw rotors are rotatably arranged in engagement with each other, at least one of the rotors being provided with at least one axle journal mounted in bearings, which bearing is oil lubricated and arranged in a bearing 10 chamber, which bearing chamber communicates with the working space via a gap seal. In a second aspect, the invention relates to an energy-conversion system comprising an expander, a condenser, a pump and an evaporator arranged in a closed conduit circuit, and a machine driven by the expander. 15 In a third aspect, the invention relates to a method for energy conversion, in which a working medium is brought to pass through a closed conduit circuit comprising an expander, a condenser, a pump and an evaporator arranged in series, the expander driving a machine. Background of the Invention 20 The invention is intended to be applied in a so-called ORC process (Organic Rankine Cycle), which is a process that in principle works as a power producing steam cycle with the difference that the working medium of the process is other than steam. Different forms of machines based on turbine technology have been used in such a cycle to be driven by the working medium, drive a generator 25 or another power-absorbing machine. However, turbines have the disadvantage of becoming very expensive at shaft powers below 1 MW.
WO 2008/121070 PCT/SE2008/050367 2 Therefore, for lower power levels, a screw expander has been used instead of a turbine, the screw expander normally being provided with oil injection, i.e., oil is injected into the working space of the expander in order to cool, seal and lubricate. 5 An oil-injected screw expander has excellent efficiency, but has some dis advantages. One disadvantage is that the oil has to be separated downstream the expansion. This means that the pressure of the working medium then is low, and hence the volumetric flow is high. Since oil separation is dependent on low flow 10 velocity, the oil separator thereby becomes impractically large and unreasonably expensive. Another disadvantage is that the types of oil separators available on the market cannot separate 100 % of all oil. This implies that a certain amount of oil passes the separator and ends up in the heat exchangers, where it seriously 15 reduces the efficiency. As alternative to an oil-injected screw expander, it would theoretically be possible to consider using the technique used in oil free screw compressors for air having internal shaft seals between the working chamber and the bearings. How ever, it is practically impossible to get these seals completely tight, and therefore it 20 is chosen to let out the leakage air into the atmosphere. However, this cannot be allowed in a closed ORC process. In addition, the internal leakages are particular difficult to check upon downtime in a pressurized system where condensation occurs against cold surfaces. The consequence would then become a successive mixing of oil and working medium, decreasing viscosity and thereby reliability 25 problems. In these circumstances, the object of the present invention is to enable energy conversion by means of an ORC process at relatively low power levels with WO 2008/121070 PCT/SE2008/050367 3 improved performance, increased reliability and with elimination of the disadvan tages associated with hitherto known technique within the field. Sumrnryof the Invention 5 The object set forth has, in the first aspect of the invention, been attained by the fact that a screw-rotor machine of the kind mentioned by way of introduction has the special features that the bearing chamber, in addition to communicating with the working space, also communicates with an oil supply conduit and a drain age conduit, which drainage conduit is connected with a pressure that is lower 10 than the lowest pressure in the working space of the machine when it is open to the outlet. With such a screw-rotor machine formed as an expander, oil free expan sion is provided at the same time as the bearings become reliably oil lubricated. Thereby, it becomes possible to utilize the same as a driving unit in an ORC proc 15 ess, cost-effectiveness being attained also at lower powers than at those where turbines are a practically feasible alternative. Thus, no oil separator is needed in the main gas flow. The advantages gained with a screw-rotor machine according to the invention are particularly valuable when it is utilized in an ORC process. The 20 advantages thereof are naturally of interest also when the machine is used as an expander in other applications where similar conditions may occur. The same applies also when the machine operates as a compressor. According to a preferred embodiment of the invented screw-rotor machine, an oil separator is arranged in the drainage conduit for separation of oil from 25 medium flowing through the drainage conduit. Thereby, the risk of oil in the heat exchanger of the ORC circuit is completely eliminated.
WO 2008/121070 PCT/SE2008/050367 4 According to an additional preferred embodiment, the oil separator is con nected to the oil supply conduit via an oil pump. Thereby, it is, in a simple way, guaranteed that the bearing lubrication is effected by a closed oil circuit, which eliminates the need of continuous supply of 5 oil, and no waste oil needs to be taken care of. According to an additional preferred embodiment, the oil separator is con nected to an evacuation pump via a conduit for working medium. Thereby, a guarantee of a required pressure difference is attained in a simple way. 10 According to an additional preferred embodiment, the oil separator is, via a connection conduit for working medium, connected to a conduit connected with the outlet opening of the screw-rotor expander. This is an embodiment that is particularly advantageous to use when the machine is utilized as an expander. As the pressure in said conduit is lower than 15 said pressure in the working chamber due to the constriction pressure losses occurring in the outlet, with this embodiment, it is automatically obtained that the stated pressure ratio is attained. An additional advantage is that the working medium flowing through the bearing chamber is brought back to the main flow of the working medium. This is particularly valuable when the expander is included in 20 an ORC process, since then it is important to avoid discharge to the surroundings of working medium, The need of supplying additional working medium is also eliminated. The object set forth has, in the second aspect of the invention, been attained by the fact that an energy-conversion system of the kind mentioned by 25 way of introduction has the special feature that the expander is a screw-rotor expander according to the present invention.
WO 2008/121070 PCT/SE2008/050367 5 By the invented energy-conversion system, profits of the advantages of the invented screw-rotor machine are derived in a context where said advantages are particularly significant. Thereby, a cost-effective and technical expedient application of an ORC 5 process becomes possible, also at relatively moderate power levels. According to a preferred embodiment of the invented energy-conversion system, the drainage conduit from the bearing chamber is connected to the circuit of the system between the expander and the pump. Then, the connection is most suitably made between the expander and the condenser. I0 According to the third aspect of the invention, the object set forth is attained by a method of the kind mentioned by way of introduction comprising the special measures of using, as an expander, a screw-rotor expander having at least two screw rotors, at least one rotor of which is mounted in at least one oil-lubri cated bearing, bringing working medium to leak out to said bearing, and leading is away mixture of oil and working medium from the bearing. By the invented method, advantages are attained of the similar type that is attained by the invented energy-conversion system and that has been accounted for above. According to a preferred embodiment of the invented method, oil is sepa 20 rated from said mixture. According to an additional preferred embodiment, the working medium is a compound based on carbon, fluorine or nitrogen. Thus, by applying the method for the type of working media that normally may be used in an ORC process, the advantages of the invented method are util 25 ized in a context where said advantages are particularly significant.
WO 2008/121070 PCT/SE2008/050367 6 According to an additional preferred embodiment, the working medium having passed the oil separator is supplied to the conduit circuit between the expander and the pump. Thereby, similar advantages as those indicated above for the corre 5 sponding embodiment of the invented energy-conversion system are gained. The invention is explained in more detail by the subsequent description of an advantageous embodiment example of the same, reference being made to the appended drawing figures. io Brief Description of the Figures Figure 1 is a diagram illustrating an energy-conversion system according to the invention. Figure 2 is a longitudinal section through a screw expander according to the invention. 15 Description of Embodiment Examples In Fig. 1, an energy-conversion system according to the invention is illus trated. The system applies an ORC process with a suitable working medium, such as, e.g., R134a, ammonia, some hydrocarbon compound or carbon dioxide. Work ing medium is supplied at a first pressure via the conduit 5 to the inlet of a screw 20 expander 1, in which the working medium expands to a lower pressure, and exits the expander 1 through the outlet conduit 6. The outlet conduit 6 directs the low-pressure working medium to a con denser 2 so that the working medium exits the same in the liquid phase through the conduit 7. By means of a pump 3, the pressure of the liquid working medium is WO 2008/121070 PCT/SE2008/050367 7 raised and is fed through conduit 8 to an evaporator 4, where the working medium is evaporated and is fed back to the inlet conduit 5 of the expander 1 at said first pressure. The expander drives a machine 9, which either may be a generator or a 5 machine that directly utilizes the mechanical energy. Thus, the working medium flows through a closed circuit and experiences phase changes by dissipation and supply, respectively, of heat. The system may be used for generation of cold, heat refinement, energy conversion of heat into mechanical energy and/or generation of electricity. 10 The system is well adapted to utilize low-grade heat, such as, e.g., from solar energy and incineration. Supplied heat is typically at a temperature lower than 140 *C, and with suitable choice of working medium, sources of heat having a temperature down to 30-40 *C may be utilized. What has been described hitherto above is previously known and is is something that is utilized today. The expander 1 is described in more detail, reference being made to Fig. 2 where the same is formed in a special way in accordance with the invention. The expander 1 comprises two screw rotors 102, only one of which is made visible in the figure. One of the rotors is of the male rotor type and the other 20 of the female rotor type, and are surrounded by a housing 101 that delimits a working space 115 having the shape of two cylinders intersecting each other. Between the rotors 102 and the housing 101, V-shaped working chambers are formed, which, when the rotors rotate, drift from the inlet 114 of the expander toward the outlet 113 thereof. The functional mode in detail of a screw-rotor 25 expander should be generally known and requires therefore, in this connection, not any further detailed description.
WO 2008/121070 PCT/SE2008/050367 8 The screw rotor 102 visible in the figure has an output shaft 103 con nected with the driven unit 9 illustrated in Fig. 1. At the other end thereof, the screw rotor is formed with an axle journal 104 mounted in a bearing 106. This is arranged in a bearing chamber 105. The bearing chamber communicates with the 5 working space of the expander 1 via narrow gap 107. An oil circuit having a supply conduit 111, a drainage conduit 108, an oil separator 109, and an oil pump 110 are connected to the bearing chamber 105. The oil separator is, via a conduit 112 for working medium, connected with the outlet conduit 6 of the expander 1. 10 In operation, gaseous working medium of relatively high pressure is sup plied to the inlet opening 114 of the expander, is expanded in the expander 1, and exits the same through the outlet 113 of the expander at a lower pressure out through the outlet conduit 6. Oil is supplied to the bearing chamber by means of the oil pump 110 15 through the supply conduit 111. The oil is then drained from the bearing chamber 105 via the drainage conduit 108 and the oil separator 109 so as to be recircu lated. The pressure conditions are such that the pressure in the bearing cham ber is lower than the part of the working space of the expander that is exposed to 20 the gap 107 between the end portion 116 of the expander and the bearing cham ber 105. Thereby, there is no risk that oil from the lubrication circuit leaks out from the bearing chamber 105 via the gap 107 into the working space 115 of the expander. Instead, there is a leakage flow of working medium from the working space 115 of the expander via the gap 107 into the bearing chamber 105, where 25 the working medium is intermixed in the oil. Hence, it is oil mixed with working medium that flows through the drainage conduit 108 to the oil separator 109. In the same, oil and working medium are separated and the latter is fed from the same via the conduit 112 to the outlet con- WO 2008/121070 PCT/SE2008/050367 9 duit of the expander, where the pressure is lower than in the working space of the expander exposed to the gap 107. Thus, the oil separator needs to be dimensioned only for the separation of the relatively limited amount of working medium that leaks out through the gap 107 5 and the limited amount of oil that circulates in the lubrication circuit for the bearing. This will be of a completely different size than in an oil-injected expander, where it is a matter of a much greater amount of oil and where all working medium is mixed with the oil. It should be appreciated that a corresponding bearing chamber may be 1o arranged also at the output shaft 103 of the expander, as well as at the axle jour nals of the other rotor.